1. Field of the Invention
The invention is directed to a method for processing the output signal of an optical earth-horizon sensor of a geostationary satellite wherein the earth-horizon sensor includes a input lens, a chopper disk which is located in the image plane of the input lens and is reciprocated with a determined amplitude (chopper amplitude) and frequency (chopper frequency) and has a diameter corresponding approximately to the image of the earth, a secondry lens and a detector, the output signal of the earth-horizon sensor being amplified and demodulated to a sensor off-course signal with the chopper frequency, the sensor off-course signal being a measurement for the off-course angle of the line of sight of the earth-horizon sensor in relation to the satellite/earth central point connecting line.
2. Description of Related Art
Control signals which indicate the deviation of a line of sight, which is fixed with respect to the satellite, in relation to the satellite-earth central point connecting line, are required for attitude control of geostationary satellites. For this purpose, optical earth-horizon sensors are used, among others, for two axes, which earth-horizon sensors can be classed with the zero-seeking sensors. Such an earth-horizon sensor works in the infrared range and is based on the mechanical vibrator or chopper principle. The infrared radiation of the earth is collected by means of an objective lens of germanium and falls on a circular chopper disk in the image plane of the lens. This chopper disk has a diameter approximately corresponding to the image of the earth and is reciprocated with a determined amplitude, the chopper amplitude, at a determined frequency, the chopper frequency. The light of the two oppositely located earth horizons alternately released with the chopper frequency, which light falls through the collecting lens and is interrupted by the chopper disk, is directed on a detector, e.g. a pyroelectric detector, via a secondary lens composed of a spherical mirror segment and a prism via a spectral filter for the infrared range. The output signal of the detector is amplified and subsequently demodulated with the chopper frequency. If the light energy received by the detector from the two earth horizons is identical, a ZERO signal is supplied by means of the demodu-ation. If this is not the case, e.g. because the line of sight of the earth-horizon sensor does not coincide with the satellite-earth central point connecting line, then the output signal of the detector depends on the difference of the received light quantities of the two horizons. This difference is a measurement of the off-course angle of the line of sight of the earth-horizon sensor in relation to the satellite-earth central point connecting line.
With such an earth-horizon sensor, the off-course angle can be indicated in one axis. Two such earth-horizon sensors are required for triaxial stabilization.
The chopper disk is reciprocated by approximately 1 mm with a chopper frequency of approximately 40 Hz and an amplitude of approximately 1/17 of the disk diameter.
Such earth-horizon sensors based on the chopper principle have several advantages compared with other sensor systems:
Firstly, the mechanical vibrator or chopper is a very simple spring-mass system which oscillates with its natural resonant frequency. Therefore, neither a drive motor nor a costly angle selection is required. Secondly, the interrupted infrared radiation is guided to only one detector per channel, thereby eliminating the problem of alignment of a plurality of detectors and the problem of weakening.
The measuring range of such an earth-horizon sensor is approximately .+-.1.degree.. The above-mentioned zero point error as a result of varying radiation of opposite earth horizons is only slight, but in extreme cases can amount to approximately .+-.16% of the indicated measuring range of a degree. An effort is made to keep this error signal produced by means of the radiation anomaly, i.e. the anomaly portion within the sensor off-course signal, as small as possible. This effort is understandable when it is taken into consideration that geostationary satellites are to be used for producing directional radio links and guided television links with the earth.